1. Field of the Invention
The present invention relates to a novel plating method which is suitably used to plate a plating target having a micropore, and a novel continuous plating method which uses means for vibrating the plating target at 10 to 60 Hz in addition to vibrational stirring means for a plating bath, aeration and swinging means for swinging the plating target which have been hitherto known. The plating method of the present invention is applicable to any method of an electroplating method, an electroless plating method and a method of using the electroplating method and the electroless plating method in combination.
2. Description of the Related Art
A printed wiring board is used as a board on which various kinds of elements are mounted to mutually wire these elements to one another in all electric equipment from radios, televisions to computers. In such a surface mount type of printed wiring board that parts are attached onto the surface of a board to be connected to one another, a multi-pin structure and a narrow-pitch design have recently propagated, and this technique has been improved more and more in such a situation that a 60-pin/1.0 mm pitch design has been developed and improved through 400-pin/0.3 mm pitch level to 400-pin/0.25 mm-0.3 mm pitch level, and further higher level.
Such an improvement of this technique has promoted a requirement for compact-size and high performance. Accordingly, the diameter of minute through holes required for the wiring between different layers on a laminate type printed wiring board and the diameter of minute holes having a dead end structure (i.e., blind holes) have been increasingly reduced, and at present the diameter is reduced from 0.2 mm or less to 0.05 mm or less, further to 0.03 mm or less.
Air, dust or treatment liquid which is trapped in these microstructured holes cannot be easily removed from the holes, and thus the following disadvantages occur. A first disadvantage resides in that portions at which air or dust exists are hardly plated, so that a conduction failure occurs. A second disadvantage resides in that entrance of plating liquid into a hole is insufficient, and thus the thickness of a plating film outside the hole is different from that inside the hole. A third disadvantage resides in that a non-plated portion occurs in a hole, so that a resistance value is increased even when conduction is established.
On the other hand, a technique disclosed in Japanese Patent Application Publication No. Hei-6-71544 which was previously developed by the inventor of this application contains a technique for vibrating a vibration plate at a vibrational amplitude of 8 to 20 mm and at a vibration frequency of 200 to 600 times per minute in a plating bath to stir the plating bath, thereby performing a plating treatment. Particularly it is proposed that aeration and swing means for a target to be treated are used in combination. The inventor of this application has considered that occurrence of plating-failure portions in the conventional plating method could be avoided if the technique disclosed in the Japanese Patent Application Publication No. Hei-6-71544 is applied to plating of minute holes (micropores) of an article having the minute holes, and has made various attempts. As a result, it has been found out that the fraction defective is surely reduced, however, it does not reach a satisfactory value.
Further, Japanese Patent Application Laid-open No. Sho-62-32690 proposes a method of plating a plating target while swinging and vibrating the plating target. It is merely disclosed that the vibration is generated by supplying air to a vibrator, however, the details such as the type of the vibration, etc. are unclear.
The inventor of this application has repetitively made tests under various combinations and conditions while varying the vibrational stirring condition of the plating bath, the aeration condition and the swinging condition of the plating target, however, satisfactory results cannot be obtained. As final means, the inventor has tried to apply to a plating target such vibrating means that is used to vibrationally stir the solution. As a result, the inventor has found out that the fraction defective can be unexpectedly remarkably reduced, and implemented the present invention.